Luna had that cough also, right after she got the bordatella vaccine
She ended up with a horrible respiratory infection that needed two rounds of antibiotics to clear up
She's fine now.
I'd talk to the vet about this, don't let it get worse. If your vet won't listen, or does the "wait and see" with you, you might want to consider a second opinion or new vet.
Good luck and I hope it's nothing.